Reflective telescope with wide field of view

ABSTRACT

Systems and methods for providing a wider FOV for a telescope system are disclosed. In one embodiment, a telescope includes a primary mirror having an orifice, where an optical path originates from an object positioned in front of the primary mirror and reflects off the primary mirror. A secondary mirror is disposed adjacent to the primary mirror, where the optical path reflects off the secondary mirror and passes through the orifice in the primary mirror. The telescope includes a set of extended field corrector optics disposed along the optical path, the extended field corrector optics positioned to reflect light incident from the secondary mirror, where the set of extended field corrector optics comprises two corrector mirrors. A tertiary mirror is disposed along the optical path and adjacent to the extended field corrector optics, the tertiary mirror positioned to reflect the light incident from the extended field corrector optics.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/139,484, filed on Mar. 27, 2015, entitled “Reflective Telescopewith Wide Field of View,” the disclosure of which is hereby incorporatedby reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Space-based telescopes are utilized by a variety of industries, such asthe defense industry and the commercial space industry. Space-basedtelescopes orbit around the Earth as they observe and/or measure theEarth's surface.

Despite the progress made in space-based telescopes, there is a need inthe art for improved methods and systems related to telescopes with awide field of view.

SUMMARY OF THE INVENTION

The present disclosure relates in general to space-based imagingsystems, and in particular to space-based telescope systems having awide field of view (FOV). The invention has wider applicability and isalso applicable to terrestrial-based imaging systems and telescopes.

Embodiments of the present invention relate to systems and methods thatprovide a wider FOV for a telescope system. In an embodiment, the systemincludes an afocal telescope that has a plurality of mirrors. Theplurality of mirrors includes a set of field corrector optics, whosepurpose is to reduce optical aberrations to an extent necessary topermit achieving a wide field of view, that are positioned to reflectlight incident from a secondary mirror. The incident light may passthrough an orifice in a primary mirror before being reflected by the setof extended field corrector optics. The invention has wide applicabilityto commercial space systems, as well as military surveillance systems,both space-based and terrestrial-based.

According to an embodiment of the present invention, a telescopeincludes a primary mirror having an orifice, where an optical pathoriginates from an object positioned in front of the primary mirror andreflects off the primary mirror. A secondary mirror may be disposedadjacent to the primary mirror, where the optical path reflects off thesecondary mirror and passes through the orifice in the primary mirror.The telescope includes a set of extended field corrector optics disposedalong the optical path, the extended field corrector optics positionedto reflect light incident from the secondary mirror, where the set ofextended field corrector optics comprises two corrector mirrors. Thetelescope further includes a tertiary mirror disposed along the opticalpath and adjacent to the extended field corrector optics.

In an embodiment, the tertiary mirror is characterized by an opticalpower suitable to collimate light received from the set of extendedfield corrector optics. Alternatively, in an embodiment, the tertiarymirror is characterized by an optical power sufficient to focus lightincident from the set of extended field corrector optics onto adetector. In certain embodiments, the tertiary mirror is positioned toreceive light incident from the set of extended field corrector opticsthat is propagating toward the primary mirror. The tertiary mirror maybe positioned adjacent to the primary mirror. In some embodiments, thetelescope further includes a fold mirror disposed along the optical pathand adjacent to the extended field corrector optics, the fold mirror maybe positioned to reflect an optical path from the tertiary mirror. Insome embodiments, the extended field corrector optics includes a convexmirror and a concave mirror. The extended field corrector optics mayhave substantially no net optical power. In an embodiment, the convexmirror and the concave mirror are disposed on opposite sides of anintermediate image. The intermediate image may be disposed equaldistances away from the convex mirror and the concave mirror. Inembodiments, the intermediate image is an accessible intermediate image.

In some embodiments, the two corrector mirrors are both aspheric. Theconvex mirror may be more aspheric than the concave mirror. In anembodiment, the convex mirror departs by up to 25 waves from a sphereand the concave mirror departs by up to 15 waves from a sphere. In someembodiments, each corrector mirror is rotationally symmetric. In someembodiments, the fold mirror and tertiary mirror are on opposite sidesof the set of extended field corrector optics. In embodiments, theprimary mirror has a center, and the orifice is offset from the center.

According to another embodiment of the present invention, a method ofcorrecting an optical field includes reflecting, by a primary mirror,light originating from an object toward a secondary mirror, andreflecting, by the secondary mirror, the light from the primary mirrortoward a set of extended field corrector optics, where the light passesthrough an orifice in the primary mirror. The method includesreflecting, by the set of extended field corrector optics, the lightfrom the secondary mirror toward a tertiary mirror, and reflecting, bythe tertiary mirror, the light from the extended field corrector opticstoward a fold mirror.

In embodiments, the set of extended field corrector optics reflects thelight twice. In some embodiments, an intermediary image is created afterthe first reflection.

According to yet another embodiment of the present invention, atelescope system includes a plurality of mirrors and a detectorpositioned to receive light incident from the plurality of mirrors. Theplurality of mirrors may include a primary mirror having an orifice,where an optical path originates from an object in front of the primarymirror and reflects off the primary mirror. A secondary mirror isdisposed adjacent to the primary mirror, where the optical path reflectsoff the secondary mirror and passes through the orifice in the primarymirror. The plurality of mirrors includes a set of extended fieldcorrector optics disposed along the optical path, the extended fieldcorrector optics positioned to reflect light incident from the secondarymirror, where the set of extended field corrector optics comprises twocorrector mirrors. The plurality of mirrors further includes a tertiarymirror disposed along the optical path and adjacent to the extendedfield corrector optics, the tertiary mirror positioned to reflect thelight incident from the extended field corrector optics. An additionalset of mirrors or lenses may then be included to focus light onto adetector.

In some embodiments, the plurality of fold mirrors further includes afold mirror disposed along the optical path and adjacent to the extendedfield corrector optics, the fold mirror positioned to reflect an opticalpath from the tertiary mirror. In some embodiments, the extended fieldcorrector optics comprises a convex mirror and a concave mirror. Inembodiments, the primary mirror has a center, and the orifice is offsetfrom the center.

Numerous benefits are achieved by way of these apparatuses overconventional apparatuses. Benefits provided by the present inventioninclude wider FOV than conventional space-based telescopes. Having awider FOV saves time and cost associated with observing the Earth fromspace. Additionally, the plurality of mirrors are arranged to have anoverall compact footprint. Having a compact footprint maximizesavailable space for other necessary payloads. Additionally, having acompact footprint saves cost associated with delivering the telescopeinto outer space.

These and other details of embodiments along with many of theiradvantages and features are described in the following description,claims, and figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an afocal telescope mirror arrangement.

FIG. 2 is a diagram illustrating an afocal telescope mirror arrangementhaving extended field corrector optics, in accordance with an embodimentof the present invention.

FIG. 3A is a diagram illustrating extended field corrector optics, inaccordance with an embodiment of the present invention.

FIG. 3B is a diagram illustrating a first corrector mirror, inaccordance with an embodiment of the present invention.

FIG. 3C is a diagram illustrating a second corrector mirror, inaccordance with an embodiment of the present invention.

FIG. 4 is a graph illustrating aspherical profiles of corrector mirrors,in accordance with an embodiment of the present invention.

FIG. 5A is a simplified plot illustrating a surface profile of the firstcorrector mirror, in accordance with an embodiment of the presentinvention.

FIG. 5B is a simplified plot illustrating a surface profile of thesecond corrector mirror, in accordance with an embodiment of the presentinvention.

FIG. 6 is a diagram illustrating a focal telescope mirror arrangementhaving extended field corrector optics, in accordance with an embodimentof the present invention.

FIG. 7A is a graphical representation of image quality measured in theSWIR band, in accordance with an embodiment of the present invention.

FIG. 7B is a graphical representation of image quality measured in thevisible band, in accordance with an embodiment of the present invention.

FIG. 8A is a map of astigmatism for an afocal telescope without extendedfield corrector optics.

FIG. 8B is a map of astigmatism for an afocal telescope with extendedfield corrector optics, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the following description, numerous examples and details are setforth in order to provide an understanding of embodiments of the presentinvention. It will be evident to one skilled in the art, however, thatcertain embodiments can be practiced without some of these details, orcan be practiced with modifications or equivalents thereof.

Embodiments of the present invention provide systems and methods for awide FOV space-based telescope. The space-based telescope may be formedof a plurality of mirrors, which may include a set of extended fieldcorrector optics. The extended field corrector optics may be positionedalong an optical path to increase the FOV of the space-based telescopesystem.

FIG. 1 illustrates a telescope 100. The telescope 100 includes a front102 and a back 104. The front 102 may be directed toward an object (notshown) from which an optical path 114 originates. Telescope 100 alsoincludes a primary mirror 106 having an orifice 107. A secondary mirror108 is positioned adjacent to the primary mirror 106. The secondarymirror 108 is positioned along the optical path to reflect lightincident from the primary mirror 106. Reflected light from the secondarymirror 108 passes through the orifice 107 and projects onto a first foldmirror 110. The first fold mirror 110 directs light toward a tertiarymirror 112, which then reflects the light toward a second fold mirror120 and additional optics as needed to form an image on a detector (notshown). The first fold mirror 110 and second fold mirror 120 are plain,flat mirrors that do not affect the image quality achieved by thetelescope 100.

As shown in FIG. 1, the first fold mirror 110, second fold mirror 120,and the tertiary mirror 112 are positioned laterally adjacent to theprimary mirror 106. The first fold mirror 110, second fold mirror 120,and the tertiary mirror 112 fit within the footprint of the primarymirror 106. Accordingly, the telescope 100 may be amenable tospace-based use because of the small footprint achieved by fitting thefirst fold mirror 110, second fold mirror 120, and the tertiary mirror112 within the boundaries of the primary mirror 106.

Table 1 below illustrates the performance of telescope 100.Specifically, Table 1 shows the percent of the collected energyensquared within a 6.25 μm pixel for the telescope 100 across ahorizontal FOV ranging from −1.25 degrees to 1.25 degrees, and avertical FOV ranging from −0.4 to 0.4 degrees. The wavelength (λ) of theincident light is between 500 to 700 nm, and the diffraction limit is71.

TABLE 1 Horizontal FOV −1.25 −0.93 −0.62 −0.31 0 0.31 0.62 0.93 1.25vertical FOV 0.4 5 21 62 57 59 57 62 21 5 0 5 31 65 55 61 55 65 31 5−0.4 3 21 56 48 46 48 56 21 3

As shown, the telescope 100 achieves energy capture of greater than 40percent within a horizontal FOV between −0.62 and 0.62. Maximumperformance of 61 percent energy capture is achieved at center. Theperformance of the telescope 100 decreases as the horizontal FOV reachesto plus or minus 0.93 degrees, and decreases even more as the horizontalFOV reaches plus or minus 1.25 degrees. Thus, telescope 100 achieves aFOV of 0.62 degrees in the horizontal direction. With respect to thevertical FOV, performance slightly decreases between −0.4 and 0.4degrees, where 59 and 46 percent energy capture is achieved at 0.4 and−0.4 degrees, respectively.

According to embodiments of the present invention, telescope performanceacross horizontal and vertical FOVs are significantly increased when afield corrector assembly, such as a set of extended field correctoroptics, is incorporated into the design, as will be discussed inrelation to FIG. 2.

FIG. 2 is a diagram illustrating an afocal telescope mirror arrangementhaving extended field corrector optics, in accordance with an embodimentof the present invention. As illustrated in FIG. 2, a telescopicarrangement producing collimated output after reflection from foldmirror 210 is provided. Some embodiments can be utilized over a range ofmagnifications, for example, between 3× and 12× although embodiments arenot limited to this particular range. FIG. 2 illustrates telescope 200having a plurality of mirrors that include a primary mirror 206 havingan orifice 207. The primary mirror 206 may have a radial center, and theorifice 207 may be positioned offset from that center. An optical path214 originating from an object (not shown) located at the front 202 ofthe telescope 200 may project toward the telescope 200. Light projectedalong the optical path 214 may reflect off the primary mirror 206,reflect off the secondary mirror 208, and then travel through theorifice 207.

According to an embodiment of the present invention, a set of extendedfield corrector optics 216 is positioned along the optical path 214 toreceive at least a portion of light incident from the secondary mirror208. In some embodiments, all of the light reflected by the secondarymirror 208 is received at the set of extended field corrector optics216. The set of extended field corrector optics may enable the telescope200 to have a wider field of view (FOV). In embodiments, the set ofextended field corrector optics 216 is positioned adjacent to theprimary mirror 206 on a side opposite of the secondary mirror 208. Forinstance, the set of extended field corrector optics 216 may bepositioned adjacent to a side of the primary mirror 206 that is facingtoward the back 204 of the telescope 200. In an embodiment, at least aportion of the extended field corrector optics may be positioned along acenter axis of the orifice 207. The set of extended field correctoroptics 216 may include a pair of mirrors, as will be discussed ingreater detail further herein with respect to FIG. 3A.

In embodiments, a tertiary mirror 212 is positioned along the opticalpath to receive at least a portion (or all) of light incident from theset of extended field corrector optics 216. The tertiary mirror 212 maybe positioned adjacent to the extended field corrector optics 216. In anembodiment, the tertiary mirror 212 is positioned below the set ofextended field corrector optics 216 and adjacent to an edge of theprimary mirror 206. In some embodiments, the tertiary mirror 212 iscurved such that the tertiary mirror 212 has an optical power suitableto collimate light received from the set of extended field correctoroptics.

As shown in FIG. 2, light incident from the set of extended fieldcorrector optics may propagate in a direction toward the primary mirror206 after passing through the set of extended field corrector optics.Accordingly, tertiary mirror 212 may be positioned to receive lightincident from the set of extended field corrector optics that ispropagating toward the primary mirror. Thus, rays are folded backtowards the primary mirror after passing through the set of extendedfield corrector optics, resulting in a compact design as discussedherein.

For instance, tertiary mirror 212 may be positioned adjacent to primarymirror 206, with the tertiary mirror closer to the back of the primarymirror than the set of extended field corrector optics. Thus, lightreflected off of tertiary mirror 212 may propagate in a space disposedbetween primary mirror 206 and the set of extended field correctoroptics 216 as it propagates toward fold mirror 210. In someimplementations, the rays propagating between the tertiary mirror andthe fold mirror are substantially parallel to the back of the primarymirror. Comparing the telescopes illustrated in FIGS. 1 and 2, thetertiary mirror is used on opposite sides of the optical axis of thetertiary mirror in FIG. 2 compared to FIG. 1. Thus, embodiments of thepresent invention provide compact designs suitable for space-based andother applications. Accordingly, some embodiments may be referred to asa compact extended field telescope.

A fold mirror 210 may be positioned along the optical path to receive atleast a portion of light incident from the tertiary mirror 212. In anembodiment, the fold mirror 210 is positioned adjacent to the extendedfile corrector optics 216. The fold mirror 210 may be positioned on aside of the set of extended field corrector optics 216 opposite thetertiary mirror 212. As illustrated in the side view of FIG. 2, thetertiary mirror is positioned below the set of extended field correctoroptics and the fold mirror is positioned above the set of extended fieldcorrector optics such that the fold mirror and the tertiary mirror areon opposite sides of the centrally located set of extended fieldcorrector optics. Light incident from the tertiary mirror 212 mayreflect off the fold mirror 210 and project toward additional opticsimaging onto a detector (not shown). The first fold mirror 110 may be aplain, flat mirror that does not affect the image quality achieved bythe telescope 200.

FIGS. 3A-3C illustrate, in more detail, the set of extended fieldcorrector optics 216 according to an embodiment of the presentinvention. More specifically, FIG. 3A is a diagram illustrating extendedfield corrector optics, in accordance with an embodiment of the presentinvention. FIG. 3B is a diagram illustrating a first corrector mirror,in accordance with an embodiment of the present invention. FIG. 3C is adiagram illustrating a second corrector mirror, in accordance with anembodiment of the present invention. Thus, FIGS. 3B and 3C illustrateprofiles of mirrors that form the set of extended field corrector opticsaccording to an embodiment of the present invention.

As shown in FIG. 3A, the set of extended field corrector optics 216 maybe formed of two corrector mirrors: a first corrector mirror 302, and asecond corrector mirror 304. The first corrector mirror 302 may bepositioned to reflect light toward the second corrector mirror 304. Forinstance, the first corrector mirror 302 may be positioned along theoptical path 214 to receive light incident from the secondary mirror 208and reflect the light toward the second corrector mirror 304. In anembodiment, the first and second corrector mirrors 302 and 304 areoptically centered. For instance, light reflecting off a rotationallysymmetric center of the first corrector mirror 302 is received by arotationally symmetric center of the second corrector 304.

According to embodiments of the present invention, light rays thatpropagate along the optical path 214 cross over one another at acrossover plane 306. For instance, the optical path 214 may be composedof three sets of light rays: a first set of light rays 312, a second setof light rays 314, and a third set of light rays 316. Each set of lightrays 312, 314, and 316 may be composed of individual light rays thatintersect one another at the crossover plane 306. In embodiments, thecrossover plane 306 is disposed substantially equidistant between thefirst and second corrector mirrors 302 and 304. Alternatively, crossoverplane 306 may be disposed slightly closer to the second corrector mirror304. In another alternative, crossover plane 306 may be disposedslightly closer to the first corrector mirror 302. Crossover plane 306may represent a point in the optical path 214 where an intermediateimage plane is formed and may be referred to as an intermediate plane.In an embodiment, the crossover/intermediate image plane is anaccessible crossover/intermediate plane. In some embodiments, a fieldstop can be placed at the crossover plane 306 to limit the field of viewof the telescope 200. In other embodiments, because the crossover planeis accessible and configured to receive optics placed at the crossoverplane, other optical elements can be placed at the crossover plane toperform other suitable optical functions. One of ordinary skill in theart would recognize many variations, modifications, and alternatives.

In embodiments, the first and second corrector mirrors 302 and 304 areconfigured to achieve little to no net paraxial optical power for thetelescope 200, i.e., substantially no net paraxial optical power. In anembodiment, the set of extended field corrector optics 216 has aparaxial optical power less than 1% of the optical power of thetelescope 200. In other embodiments, the paraxial optical power of theset of extended field corrector optics is less than 0.8%, 06%, 0.4%, or0.2% of the optical power of the telescope 200. As an example, the firstcorrector mirror 302 may have light reflecting properties that areopposite to the light reflecting properties of the second correctormirror 302. For instance, the first corrector mirror 302 may be a convexmirror, as illustrated in FIG. 3B, and the second corrector mirror 304may be a concave mirror, as illustrated in FIG. 3C. In some embodiments,the first and second corrector mirrors 302 and 304 may be configured toprovide additional paraxial optical power to the design.

The first and second corrector mirrors 302 and 304 may have a firstcorrector reflective surface 310 and a second corrector reflectivesurface 308, respectively, both of which may operate as the lightreflecting surface. In an embodiment, the first corrector reflectivesurface 310 and the second corrector reflective surface 308 are disposedtoward the crossover plane 306. In embodiments, the first and secondcorrector mirrors 302 and 304 are each rotationally symmetrical about acenter location, which may not be the physical center of the mirrors 302and 304. Although FIG. 3A illustrates the first corrector mirror 302 asthe convex mirror and the second corrector mirror 304 as the concavemirror, embodiments where the first corrector mirror 302 is concave andthe second corrector mirror 304 is convex are envisioned in alternativeembodiments herein as well.

In an embodiment, the first and second corrector mirrors 302 and 304 areaspherical mirrors. Their deviation from a sphere may be minor, as shownin FIG. 4, which illustrates a graphical representation of the curvatureof the first and second corrector mirrors 302 and 304. The graph of FIG.4 has a vertical axis representing waves of sag at 632.8 nm, and ahorizontal axis representing normalized aperture height where 0represents the center of the mirror, and 1 represents the edge of themirror. A first corrector curve 402 represents the mirror profile of thefirst corrector mirror 302, and the second corrector curve 404represents the mirror profile of the second corrector mirror 304. In anembodiment, the first corrector mirror 302 may be more aspherical thanthe second corrector mirror 304. For instance, the first correctormirror 302 may deviate from a spherical profile by at most 25 waves,whereas the second corrector mirror 304 may deviate from a sphericalprofile by at most 15 waves.

FIG. 5A is a simplified plot illustrating a surface profile of the firstcorrector mirror according to an embodiment of the present invention. Asillustrated in FIG. 5A, the surface profile of the mirror varies from aspherical profile by a few fractions of a mil (thousandth of an inch).Referring to FIG. 5A, the surface profile has a sagitta that departsfrom a sphere as a function of the normalized surface height. Althoughcharacterized by a spherical sag at the edges of the mirror, the surfaceprofile has less sag than a spherical mirror, with a maximum departurefor a spherical surface of about 0.2×10⁻³ inches near the location abouthalf the normalized surface height. As discussed above, the actual shapeof the curve defining the departure from a spherical mirror will dependon the surface profile of the second corrector mirror. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

FIG. 5B is a simplified plot illustrating a surface profile of thesecond corrector mirror according to an embodiment of the presentinvention. As illustrated in FIG. 5B, the surface profile of the mirrorvaries from a spherical profile by a few fractions of a mil (thousandthof an inch). Referring to FIG. 5B, the surface profile has a sagittathat departs from a sphere as a function of the normalized surfaceheight. Although characterized by a spherical sag at the edges of themirror, the surface profile has less sag than a spherical mirror(measured in terms of negative sag), with a maximum departure for aspherical surface of about 0.3×10⁻³ inches near the location of abouthalf the normalized surface height. As discussed above, the actual shapeof the curve defining the departure from a spherical mirror will dependon the surface profile of the first corrector mirror. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

Table 2 below illustrates the performance of a telescope according to anembodiment of the present invention. Specifically, Table 2 shows thepercent of collected energy captured within a 6.25 μm pixel for thetelescope 200 across a horizontal FOV ranging from −1.25 degrees to 1.25degrees, and a vertical FOV ranging from −0.4 to 0.4 degrees. Thewavelength (λ) of the incident light is between 500 to 700 nm, and thediffraction limit is 71. It is to be appreciated that the configurationof Table 2 is arranged such that it is similar to the configuration ofTable 1 for ease of comparison.

TABLE 2 Horizontal FOV −1.25 −0.93 −0.62 −0.31 0 0.31 0.62 0.93 1.25vertical FOV 0.4 44 66 63 65 67 65 63 66 44 0 64 61 62 67 70 67 62 61 64−0.4 57 61 63 64 65 64 63 61 57

As shown, telescope 200, according to embodiments of the presentinvention, achieves energy capture of greater than 40 percent within ahorizontal FOV between −1.25 and 1.25. Maximum performance of 70 percentenergy capture is achieved at center. The performance of the telescope200 does not significantly decrease as the horizontal FOV reaches toplus or minus 0.93 degrees, nor does it decrease below 40 percent as thehorizontal FOV reaches plus or minus 1.25 degrees. Thus, telescope 200according to embodiments of the present invention, can achieve a FOV of1.25 degrees in the horizontal direction, which is substantially greaterthan the FOV of 0.62 degrees of telescopes 100. With respect to thevertical FOV, performance slightly decreases between −0.4 and 0.4degrees, where 67 and 65 percent energy capture is achieved at 0.4 and−0.4 degrees, respectively, which is still greater than 59 and 46percent energy capture at 0.4 and −0.4 degrees, respectively, fortelescopes 100.

Tables 3 and 4 below further illustrate the performance of a telescopeaccording to embodiments of the present invention. Table 3 shows thepercent of collected energy captured within a 2.45 μm pixel for thetelescope 200 in the visible band with a 500 mm focal length perfectimager across a horizontal FOV ranging from −1.25 degrees to 1.25degrees, and a vertical FOV ranging from −0.5 to 0.5 degrees. Thediffraction limit is 26.

TABLE 3 Horizontal FOV −1.25 −0.93 −0.62 −0.31 0 0.31 0.62 0.93 1.25vertical FOV 0.5 13 25 25 25 25 25 25 25 13 0 25 24 25 25 25 25 25 24 25−0.5 22 22 25 25 25 25 25 22 22

Table 4 shows the percent of collected energy captured within a 18 μmpixel for the telescope 200 in the short wave infrared (SWIR) band witha 500 mm focal length perfect imager across a horizontal FOV rangingfrom −1.25 degrees to 1.25 degrees, and a vertical FOV ranging from −0.5to 0.5 degrees. The diffraction limit is 80.

TABLE 4 Horizontal FOV −1.25 −0.93 −0.62 −0.31 0 0.31 0.62 0.93 1.25vertical FOV 0.5 75 80 80 80 80 80 80 80 75 0 79 80 80 80 80 80 80 80 79−0.5 79 79 80 80 80 80 80 79 79

As shown in both Tables 3 and 4, maximum energy capture performance fortelescope 200 is achieved at center and stays substantially constantacross a vast majority of the field of view. The performance of thetelescope 200 does not significantly decrease until the very edges ofthe field of view, e.g., plus and minus 1.25 degrees. As can bedetermined from Tables 3 and 4, telescope 200 can achieve high energycapture across a wide FOV.

In addition to Tables 3 and 4, proof of the effectiveness of embodimentsof the present invention can also be seen from FIGS. 7A-7B, whichillustrate a graphical representation of image quality, as measured bymodulation transfer function (MTF), produced by an afocal telescopedesign, such as the telescope design of telescope 200 discussed herein.The graphs of FIGS. 7A-7B have a vertical axis representing modulation,and a horizontal axis representing spatial frequency in terms ofcycles/mm.

FIG. 7A shows the image quality measured in the SWIR band (wavelengthsbetween 1400-1800 nm) wherein plotted curve 710 represents the MTFperformance of telescopes according to embodiments of the presentinvention both at 0 degree FOV and the maximum design FOV. As can beseen from FIG. 7A, modulation is greatest at a spatial frequency of 0and decreases to approximately 0.1 at a spatial frequency of 80. 0.5modulation is attained at a spatial frequency of 40.

FIG. 7B shows the image quality measured in the visible band(wavelengths between 500-700 nm) where plotted curve 750 represents theMTF performance of telescopes according to embodiments of the presentinvention at 0 degree FOV, and plotted curve 760 represents the MTFperformance of telescopes according to embodiments of the presentinvention at the maximum design FOV. As shown in FIG. 7B, modulation forcurves 750 and 760 is greatest at a spatial frequency of 0, anddecreases to approximately 0.5 and 0.3, respectively at a spatialfrequency of 100.

FIGS. 8A-8B illustrate maps of astigmatism across a FOV for telescopeswith and without field corrected designs to show how telescopes withfield extended field corrector optics improve over telescopes withoutextended field corrector optics. Specifically, FIG. 8A illustrates a mapof astigmatism across a FOV for a telescope without extended fieldcorrector optics (e.g., telescope 100) and FIG. 8B illustrates a map ofastigmatism across a FOV for a telescope with extended field correctoroptics (e.g., telescope 200). As can be seen when contrasting FIG. 8Awith 8B, the degree of astigmatism, as represented by the lengths of thevector lines, is substantially less than telescopes without extendedfield corrector optics across a vast majority of the FOV. Lessastigmatism results in more accurate and less distorted images that canbe captured by the telescope.

FIG. 6 is a diagram illustrating a telescope 600 having a focaltelescope mirror arrangement that includes extended field correctoroptics in accordance with an embodiment of the present invention. Asillustrated in FIG. 6, a reflective telescopic arrangement is providedin which light is focused to an image at the output of fold mirror 610,thereby providing an imaging version in this embodiment. As discussedpreviously in relation to FIG. 2, some embodiments provide an afocaltelescope system that produces collimated (i.e., parallel) light at theoutput of the system. However, in alternative embodiments as illustratedin FIG. 6, the set of extended field corrector optics 616 can be usedfor an imaging reflective system instead of a true afocal telescope asdiscussed in relation to FIG. 2. As shown in FIG. 6, a tertiary mirror612 having a deeper curvature than that of the tertiary mirror 212 ofFIG. 2 may be utilized in the telescope 600. Accordingly, the mirrorarrangement would focus light directly and could be used with a detector618 without the need for additional focusing optics. In someembodiments, additional focusing optics are utilized, for example, byutilizing a fold mirror with optical power. One of ordinary skill in theart would recognize many variations, modifications, and alternatives.

Some similarities are shared with the embodiment illustrated in FIG. 2,including the primary mirror 606, the secondary mirror 608, and the setof extended field corrector optics 616. As illustrated in FIG. 6, theprimary mirror has a center and the orifice 607 can be offset from thecenter. Because of the focal nature of telescope 600, the opticalspecifications/prescriptions for the optical elements, including theprimary mirror 606, the secondary mirror 608, the tertiary mirror 612,and the first corrector mirror and the second corrector mirror includedin the set of extended field corrector optics 616, may differ from theoptical specifications for similar elements in telescope 200. Thus, aswill be evident to one of skill in the art, in a manner similar to thedesign of an afocal system as illustrated in FIG. 2, the focal systemillustrated in FIG. 6 will utilize optical elements that are modified asneeded, particularly the focusing power of tertiary mirror 612, toprovide a focal system. One of ordinary skill in the art of opticaldesign will accordingly be able to specify the optical characteristicsuitable for either afocal or focal designs.

The tertiary mirror 612 is characterized by a predetermined opticalpower suitable to form an image at the output, which can be the imagingplane of detector 618. In some embodiments as mentioned above, the foldmirror 610 can have a second predetermined optical power although thisis not required by the present invention. As discussed in relation toFIG. 2, the set of extended field corrector optics 616 can havesubstantially no net optical power. In other embodiments, thecombination of tertiary mirror 612, extended field corrector optics 616,and/or the fold mirror 610 provide the optical power to focus the image,for example, at detector 618.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. For example, although certain embodiments have beendescribed with respect to particular process flows and steps, it shouldbe apparent to those skilled in the art that the scope of the presentinvention is not strictly limited to the described flows and steps.Steps described as sequential may be executed in parallel, order ofsteps may be varied, and steps may be modified, combined, added, oromitted. As another example, although certain embodiments have beendescribed using a particular combination of hardware and software, itshould be recognized that other combinations of hardware and softwareare possible, and that specific operations described as beingimplemented in software can also be implemented in hardware and viceversa.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than restrictive sense. Other arrangements,embodiments, implementations and equivalents will be evident to thoseskilled in the art and may be employed without departing from the spiritand scope of the invention as set forth in the following claims.

What is claimed is:
 1. A telescope comprising: a primary mirror havingan orifice, wherein an optical path originates from an object positionedin front of the primary mirror and reflects off the primary mirror; asecondary mirror disposed adjacent to the primary mirror, wherein theoptical path reflects off the secondary mirror and passes through theorifice in the primary mirror; a set of extended field corrector opticsdisposed along the optical path, the extended field corrector opticspositioned to reflect light incident from the secondary mirror, whereinthe set of extended field corrector optics comprises two correctormirrors; and a tertiary mirror disposed along the optical path andadjacent to the extended field corrector optics, the tertiary mirrorpositioned to reflect the light incident from the extended fieldcorrector optics.
 2. The apparatus of claim 1 wherein the tertiarymirror is positioned to receive light incident from the set of extendedfield corrector optics that is propagating toward the primary mirror. 3.The apparatus of claim 2 wherein the tertiary mirror is positionedadjacent to the primary mirror.
 4. The apparatus of claim 1 furthercomprising a fold mirror disposed along the optical path and adjacent tothe extended field corrector optics, the fold mirror positioned toreflect an optical path from the tertiary mirror.
 5. The apparatus ofclaim 1 wherein the extended field corrector optics comprises a convexmirror and a concave mirror.
 6. The apparatus of claim 5 wherein theextended field corrector optics has substantially no net paraxialoptical power.
 7. The apparatus of claim 5 wherein the convex mirror andthe concave mirror are disposed on opposite sides of an intermediateimage.
 8. The apparatus of claim 7 wherein the intermediate image isdisposed equal distances away from the convex mirror and the concavemirror.
 9. The apparatus of claim 7 wherein the intermediate image is anaccessible intermediate image.
 10. The apparatus of claim 1 wherein thetwo corrector mirrors are both aspheric.
 11. The apparatus of claim 10wherein the convex mirror is more aspheric than the concave mirror. 12.The apparatus of claim 1 wherein the fold mirror and tertiary mirror areon opposite sides of the set of extended field corrector optics.
 13. Theapparatus of claim 1 wherein the primary mirror has a center, and theorifice is offset from the center.
 14. A method of correcting an opticalfield, the method comprising: reflecting, by a primary mirror, a lightoriginating from an object toward a secondary mirror, reflecting, by thesecondary mirror, the light from the primary mirror toward a set ofextended field corrector optics, wherein the light passes through anorifice in the primary mirror; reflecting, by the set of extended fieldcorrector optics, the light from the secondary mirror toward a tertiarymirror; and reflecting, by the tertiary mirror, the light from theextended field corrector optics toward a fold mirror.
 15. The method ofclaim 14 wherein the set of extended field corrector optics reflects thelight twice.
 16. The method of claim 15 wherein an intermediary image iscreated after the first reflection.
 17. A telescope system comprising: aplurality of mirrors comprising: a primary mirror having an orifice,wherein an optical path originates from an object positioned in front ofthe primary mirror and reflects off the primary mirror; a secondarymirror disposed adjacent to the primary mirror, wherein the optical pathreflects off the secondary mirror and passes through the orifice in theprimary mirror; a set of extended field corrector optics disposed alongthe optical path, the extended field corrector optics positioned toreflect light incident from the secondary mirror, wherein the set ofextended field corrector optics comprises two corrector mirrors; and atertiary mirror disposed along the optical path and adjacent to theextended field corrector optics, the tertiary mirror positioned toreflect the light incident from the extended field corrector optics; anda detector positioned to receive light incident from the plurality ofmirrors.
 18. The system of claim 17 wherein the plurality of mirrorsfurther comprises a fold mirror disposed along the optical path andadjacent to the extended field corrector optics, the fold mirrorpositioned to reflect an optical path from the tertiary mirror.
 19. Thesystem of claim 17 wherein the extended field corrector optics comprisesa convex mirror and a concave mirror.
 20. The system of claim 17 whereinthe light incident from the plurality of mirrors received at thedetector comprises an image of the object.